This invention relates generally to NMR spectroscopy and particularly to the data acquisition and control aspects of NMR spectroscopy apparatus.
NMR spectroscopy by the Fourier transform method has proven to be a prolific analytical technique. Such instrumentation virtually compels a computer-based system in order to accommodate the requirements of the method and to retain flexibility. A modern Fourier transform spectrometer has the capability to supply excitation pulses to the sample under study which may vary in phase, duration, interval between pulses; auxiliary transients such as decoupling pulses, homogeneity spoiling pulses, and control functions such as receiver and transmitter gating, analog-to-digital conversion commands and the like. These operations provide only for the bare acquisition of data. Time averaging, storage, transformation to the time domain, background analysis, online display and hard copy output are another class of operations which necessarily place demands on computational effort. A still further class of data handling is encountered in the evaluation and interpretation of data, comparison of data to theory and other high-level operations.
In the prior art, these classes of operations have been most commonly accommodated by an interrupt-based system architecture employing a processor which was interrupted in its current task by a task of higher priority requiring servicing, after which the processor restored conditions ante interrupt and returned to the interrupted task. The efficiency of such apparatus is never maximal for any task regardless of the priority assignment and considerable effort is required to establish and maintain such a system.
Fourier transform spectrometers achieve statistical precision by a large number of repetitions of the excitation-detection cycle. The currently acquired digitized data increments (actually a digitized waveform) are added to the previously acquired signals at addresses corresponding to values of the independent variable (time). Thus an averaged time domain spectrum (or averaged waveform) evolves in the memory. The introduction of semiconductor random access memory (RAM) elements makes it economically feasible to provide fast, large memories for this purpose. While the economy and technical specifications for these components are desirable, contemporary semiconductor RAM is volatile an does not retain information in the event of power failure or severe power transients. As a consequence, very long data acquisition runs for the measurement of low intensity spectral features may result in a complete loss of data in the event of power disturbance. Ordinarily, standby battery power is required to preserve the memory content under such conditions. This in turn requires periodic maintenance to insure viability and additionally adds to the original and operating costs of the instrument.
In the prior art, pulse sequence generation for NMR spectroscopy has been implemented by a digital computer employing both software and hardware elements cooperating in varying degrees to present the gating signal to the desired device. The gate signal from the computer would have the desired time separation from the preceding gate and the desired persistence interval. The combination of the control function with the processing of the NMR response has been commonly implemented with a priority interrupt based computer system. Distinct limitations in both data acquisition rate capability and further computational operations on the data characterize such systems. A general discussion of such apparatus may be found in Shaw, "Fourier Transform N.M.R. Spectroscopy", (Elsevier Scientific Publishing Company, 1976). In addition, at high incidence of interrupts, difficulties are encountered in maintaining the synchronism of the excitation and waveform digitization with respect to the time reference.